Spatial and temporal patterns of growth, erosion, productivity, and morphology of the dominant habitat-forming kelp Ecklonia radiata (C. Agardh) J. Agardh were studied bimonthly over 1.5 years in a southern New Zealand fjord characterized by strong gradients in light and wave exposure. Spatial differences in growth were observed with rates at two outer coast, high-light, wave-exposed sites reaching 0.42 and 0.45 cm · d−1, respectively, compared to 0.27 cm · d−1 at an inner, more homogeneous site. Sporophyte productivity was similar among sites, although population productivity was greater at the outer sites due to population density being 5-fold greater than at the inner site. It was expected that the inner site would have no pronounced seasonal pattern in growth and productivity due to its homogeneity; however, all three sites displayed maximum rates in late winter/spring and minimal in autumn. Growth rates were 2-fold greater during the first growth period than the following year. This discrepancy was not correlated to inorganic nitrogen (N) levels, which remained low year-round (<4 μM), and is likely a result of an interaction between light and temperature, and the photosynthetic capability of E. radiata. Variable pigment content indicated photoacclimation at the inner site. Morphological differences were observed between sites, with E. radiata from the inner site having longer, wider, thinner blades and longer stipes. While E. radiata displayed spatial differences in growth, erosion, productivity, and morphology, populations displayed no temporal differences. These results highlight the need for greater understanding of the mechanisms influencing kelp growth and productivity in a unique marine environment.